This invention relates to molecular sieves and processes for their manufacture. More especially it relates to processes in which synthesis mixtures are seeded to control process conditions and product characteristics. The invention relates primarily to the manufacture of zeolites and other crystalline molecular sieves. Examples of the latter include phosphorus-containing molecular sieves whether or not they have zeolite analogues.
It is well-known that seeding a molecular sieve synthesis mixture frequently has beneficial effects, for example in controlling the particle size of the product, avoiding the need for an organic template, accelerating synthesis, and improving the proportion of product that is of the intended structure type. Colloidal seeds have proved especially effectivexe2x80x94see, for example, International Application Nos. WO 97/03020 and 03021, and EP-A-753483, 753484 and 753485.
Whereas procedures for the preparation of colloidal dispersions of certain structure types have been described in the above-mentioned references, and similar procedures are effective in the preparation of colloidal dispersions of crystalline molecular sieves of other structure types, these procedures have proved ineffective in the preparation of colloidal dispersions of certain further structure types, especially LEV.
As used in this specification, the term xe2x80x9cstructure typexe2x80x9d is used in the sense described in the Structure Type Atlas, Zeolites 17, 1996.
It has now been found that for many structure types a process for manufacturing a crystalline molecular sieve produces a product of a desired, larger, particle size, which 35 particles have much smaller particles, of a size suitable for use as seeds in subsequent manufacturing processes, adhering loosely to them.
The present invention accordingly provides in a first aspect a process for the manufacture of seed crystals of a molecular sieve, which comprises synthesizing the molecular sieve by treatment f an appropriate synthesis mixture, separating from the treated synthesis mixture a crystalline molecular sieve comprising particles of a first, larger, particle size in admixture with particles of a second, smaller, size suitable for use as seed crystals, and treating the crystalline molecular sieve to separate the larger particles from the smaller particles.
In a second aspect, the invention provides a process for the manufacture of a crystalline molecular sieve by treatment of a synthesis mixture appropriate for the formation of that molecular sieve, wherein the mixture contains as seeds separated smaller particles obtainable by, and preferably obtained by, the process of the first aspect of the invention.
In a third aspect, the invention provides the use of seed crystals obtainable by, and preferably obtained by, the process of the first aspect to accelerate the rate of production of a crystalline molecular sieve by treatment of a synthesis mixture.
In a fourth aspect, the invention provides the use of seed crystals obtainable by, and preferably obtained by, the process of the first aspect to control a characteristic, for example the purity, the phase purity, the particle shape, the particle size, or the particle size distribution, of a crystalline molecular sieve produced by treatment of a synthesis mixture.
In a fifth aspect, the invention provides the use of seed crystals obtainable by, and preferably obtained by, the process of the first aspect to facilitate the manufacture of a crystalline molecular sieve by treatment of a synthesis mixture substantially free from organic structure-directing agent (template).
In a sixth aspect, the invention provides the use of seed crystals obtainable by, and preferably obtained by, the process of the first aspect to facilitate the manufacture of a crystalline molecular sieve by treatment of a synthesis mixture, without stirring, at least after the desired synthesis temperature has been reached.
Referring now in more detail to the first aspect of the invention, it will be appreciated that it is applicable to all crystalline molecular sieve structure types, and to all processes for the manufacture of a crystalline molecular sieve of such a structure type, in which the initial product of synthesis is a product containing smaller particles adhering to the larger particles. To establish applicability requires only a simple routine experiment. In one such routine experiment, which is also a preferred method of obtaining the seed crystals, the synthesis mixture containing the crystalline molecular sieve product is centrifuged and the solids washed in, advantageously deionized, water, a two-stage procedure which is repeated a number of times. If the first aspect of the invention is applicable, the supernatant water after washing will not be clear.
It has been observed that in some systems while the first wash water may sometimes be clear, and may contain no or very few dispersed crystalline molecular sieve particles, the second or subsequent wash water is in contrast not clear, and has a measurable solids content.
The procedure yields hazy supernatants after various numbers of repetitions (depending both on the system and the relative sizes of the sample and the washing water); with some systems as many as 8 may be required; 2 to 5 is typical.
Among the structure types to which the first aspect of the invention is applicable, there may be mentioned LEV, FER, TON, MFI, MFS and MOR.
Among the specific examples within the structure types, there may be mentioned Levyne, ZK-20, NU-3 and ZSM-45 (LEV), ferrierite, ZSM-21, ZSM-35, ZSM-38, NU-23, FU-9, or ISI-6, (FER), ZSM-22, NU-10, ISI-1 or KZ-2 (TON), TS-1 (MFI), ZSM-57 (MFS) and Mordenite (MOR). Using the specific examples of the products of the first aspect of the invention, there may be prepared, in the remaining aspects, those specific examples and, in addition, many others.
As indicated above, separation of the smaller particles, hereinafter termed xe2x80x9cwashwater seedsxe2x80x9d, from the larger particles may be carried out by repeated washing of the crystalline product obtained from the synthesis mixture until the supernatant wash water is hazy. Advantageously, the seeds are recovered not earlier than the second wash to limit contamination by unreacted starting materials remaining in the synthesis mixture, and preferably the suspension of washwater seeds is substantially free of such materials.
Other separations may be effected by subjecting the synthesis mixture to fractionation, low speed centrifuging, gel permeation, surfactant treatment, ammonia treatment, or: a combination of the two last mentioned.
(Although separation is advantageously complete, it is, within the scope of the invention to produce washwater seeds admixed with a small proportion of the larger particles.)
The washwater seeds, however separated, are found to have particle sizes in the range 20 to 500 nm (the smallest dimension being measured), and as such can be regarded as colloidal. The particle size of the recovered seeds may be controlled by, for example, varying the speed of the centrifuge. The seeds are advantageously used in the form of a dispersion in the separating medium, advantageously water although, in a presently less preferred alternative, they may be dried and added to a subsequent synthesis mixture in any form, provided they are not treated in any way, for example calcining, that reduces their seeding activity.
As used herein, the term xe2x80x9ccolloidalxe2x80x9d, when used of a suspension, refers to one containing discrete finely divided particles dispersed in a continuous liquid phase and preferably refers to a suspension that is stable, in the sense that no visible separation occurs or sediment forms, in a period sufficient for the use intended, advantageously for at least 10, more advantageously at least 20, preferably at least 100, and more preferably at least 500, hours at ambient temperature (23xc2x0 C.)
In each of the second and subsequent aspects of the invention, the washwater seeds are incorporated in a In synthesis mixture that is otherwise as known in the art or as described in the literature for the production of the molecular sieve concerned. This is also the case for the conditions of treatment, except that the use of washwater seeds makes possible reduced reaction times and may obviate stirring if that were otherwise necessary.
The seeds are advantageously stirred into the synthesist mixture for a time sufficient to provide a uniform dispersion, this time being dependent primarily on the viscosity of the synthesis mixture, but ranging generally from 30 seconds to 10 minutes.
The concentration of seeds in the washwater may advantageously be within the range of 0.001% to 20%, preferably within the range of 0.01% to 0.15%, and most preferably from 0.05 to 0.1%, by weight. The washwater is advantageously added to the subsequent synthesis mixture in;
such a proportion that the synthesis mixture contains the seeds at a concentration of up to 10000, advantageously at most 3000, more advantageously at most 1500, and preferably at most 1000, more preferably at most 500, and most preferably at most 350 ppm, based on the total weight of the synthesis mixture. A minimum seeding level is generally 1 ppb (0.001 ppm), advantageously at least 0.1, more advantageously at least 1, and preferably at least 10, ppm, based on the total weight of the synthesis mixture. Advantageous ranges of proportions are from 1 to 2000, preferably 100 to 1500, and most preferably 100 to 350, ppm.
In general, the seeds will be of the same molecular sieve structure type as the desired product of the second and subsequent aspects of the invention, and in many cases the seeds and the product will be the same molecular sieve, although not necessarily of identical composition.
In general, the treatment of the synthesis mixture to yield the desired crystalline molecular sieve, usually termed hydrothermal treatment, though strictly that term should be used only for treatments in which there is vapour-phase water present, is advantageously carried out under autogenous pressure, for example in an autoclave, for example a stainless steel autoclave which may, if desired, be ptfe-lined. The treatment may, for example, be carried out at a temperature within the range of from 50, advantageously from 90, especially 120, to 250xc2x0 C., depending on the molecular sieve being made. The treatment may, for example, be carried out for a period within the range of from 20 to 200 hours, preferably up to 100 hours, again depending on the molecular sieve being formed. The procedure may include an ageing period, either at room temperature or, preferably, at a moderately elevated temperature, before the hydrothermal treatment at more elevated temperature. The latter may include a period of gradual or stepwise variation in temperature.
For certain applications, the treatment is carried out with stirring or with rotating the vessel about a horizontal axis (tumbling). For other applications, static hydrothermal treatment is preferred. If desired, the synthesis mixture may be stirred or tumbled during an initial part of the heating stage, for example, from room temperature to an elevated, e.g., the final treatment, temperature, and be static for the remainder. Agitation generally produces a product with a smaller particle size and a narrower particle size distribution than static hydrothermal treatment.
The invention also provides the products of the processes and of the uses of the earlier aspects of the invention. In addition to their use as seed crystals, the washwater seeds, re-suspended after drying or preferably from their as-manufactured suspension, may be used in the manufacture of molecular sieve, especially zeolite, supported layers or membranes, for example those described in International Application No. WO 94/25151, as may the products of the remaining aspects of the invention. Other uses for the washwater seeds include all those for which colloidal seeds are suitable. The products of the remaining aspects of the invention, if required after cation exchange and/or calcining, have utility as catalyst precursors, catalysts, and separation and absorption media. They are especially useful in numerous hydrocarbon conversions, separations and absorptions. They may be used alone, or in admixture with other molecular sieves, in particulate form, supported or unsupported, or in the form of a supported layer, for example in the form of a membrane, for example as described in WO 94/25151. Hydrocarbon conversions include, for example, cracking, reforming, hydrofining, aromatization, oligomerisation, isomerization, dewaxing, and hydrocracking (e.g., naphtha to light olefins, higher to lower molecular weight hydrocarbons, alkylation, transalkylation, disproportionation or isomerization of aromatics) Other conversions include the reaction of alcohols with olefins and the conversion of oxygenates to hydrocarbons.
Conversion of oxygenates may be carried out with the oxygenate, e.g., methanol, in the liquid or, preferably, the vapour phase, in batch or, preferably, continuous mode. When carried out in continuous mode, a weight hourly space velocity (WHSV) based on oxygenate, of advantageously 1 to 1000, preferably 1 to 100, hourxe2x88x921 may conveniently be used. An elevated temperature is generally required to obtain economic conversion rates, e.g., one between 300 and 600xc2x0 C., preferably from 400 to S00xc2x0 C., and more preferably about 450xc2x0 C. The catalyst may be in a fixed bed, or a dynamic, e.g., fluidized or moving, bed.
The oxygenate feedstock may be mixed with a diluent, inert under the reaction conditions, e.g., argon, nitrogen, carbon dioxide, hydrogen, or steam. The concentration of methanol in the feedstream may vary widely, e.g., from 5 to 90 mole per cent of the feedstock. The pressure may vary within a wide range, e.g., from atmospheric to 500 kPa.